The present invention relates to a novel method for producing polyimide resins applicable to heat resistant films, heat resistant molded articles, adhesives, etc. without using solvents.
Polyimide resins have hitherto been obtained by reacting a tetracarboxylic acid dianhydride with a diamine in an organic solvent to produce a polyamic acid solution, and further cyclizing it by heating or chemically. According to this method, organic solvents of high polarity in which the polyamic acid is soluble must generally be used. However, most of them are expensive and harmful, and, besides, many production steps are needed. Furthermore, the tetracarboxylic acid dianhydride used contains impurities subjected to ring opening upon reacting with water or reacts with water in the air before use to undergo ring opening to produce the corresponding tetracarboxylic acid and the monohydride thereof and lose reactivity. Therefore, tetracarboxylic acid dianhydrides of high purity must be procured, and purity must be maintained for protecting from influence of water. Similarly, many of the reaction solvents readily absorb water in the air to cause deterioration in reactivity of the tetracarboxylic acid dianhydrides during the reaction. Thus, expensive organic solvents of high purity must be procured and absorption of water must be inhibited.
As a result of intensive research in an attempt to solve the problems in the conventional method for synthesis of polyimide resins which has been performed through production of polyamic acids using organic solvents, the present invention has been accomplished. The object of the present invention is to provide a synthesis method according to which polyimide resins excellent in heat resistance and mechanical strength can be obtained easily by use of inexpensive starting materials.
That is, the present invention is a method for producing polyimide resins which comprises mixing a diamine and at least one tetracarboxylic acid component selected from the group consisting of a tetracarboxylic acid, a tetracarboxylic acid monoanhydride and a tetracarboxylic acid dianhydride capable of forming two imide rings upon cyclization without using solvents, and then heat-treating the mixture.
As the tetracarboxylic acid dianhydride used in the present invention, there may be used, for example, aromatic tetracarboxylic acid dianhydrides such as pyromellitic acid dianhydride, biphenyltetracarboxylic acid dianhydride, benzophenonetetracarboxylic acid dianhydride, oxydiphthalic acid dianhydride and 4,4xe2x80x2-(hexafluoroisopropylidene)diphthalic acid dianhydride, and aliphatic tetracarboxylic acid dianhydrides such as cyclobutanetetracarboxylic acid dianhydride and cyclopentanetetracarboxylic acid dianhydride. The tetracarboxylic acid dianhydrides used in the present invention are not limited to these examples. They can be used each alone or in combination of two or more.
The tetracarboxylic acids and tetracarboxylic acid monoanhydrides capable of forming two imide rings upon cyclization are obtained by reacting the above tetracarboxylic acid dianhydrides with water to cause ring opening. They may be derived from either aromatic tetracarboxylic acid dianhydrides or aliphatic tetracarboxylic acid dianhydries.
This means that in the present invention the tetracarboxylic acid dianhydrides may contain, as impurities, partly ring-opened monoanhydrides or tetracarboxylic acids. Moreover, these may be positively added to the tetracarboxylic acid dianhydrides. In the conventional solution reaction using a polyamic acid as an intermediate product, unless tetracarboxylic acid dianhydrides of high purity in polyimide grade are used, reaction rate decreases and sufficient characteristics of cured products cannot be obtained. On the other hand, according to the present invention, inexpensive tetracarboxylic acid dianhydrides of low purity can also be used.
As diamines used in the present invention, mention may be made of, for example, 4,6-dimethyl-m-phenylenediamine, 2,5-dimethyl-p-phenylenediamine, 2,4-diaminomesitylene, 4,4xe2x80x2-methylenedi-o-toluidine, 4,4xe2x80x2-methylenedi-2,6-xylidine, 4,4xe2x80x2-methylene-2,6-diethylaniline, 2,4-toluenediamine, m-phenylenediamine, p-phenylenediamine, 4,4xe2x80x2-diaminodiphenylpropane, 3,3xe2x80x2-diaminodipenylpropane, 4,4xe2x80x2-diaminodiphenylethane, 3,3xe2x80x2-diaminodiphenylethane, 4,4xe2x80x2-diaminodiphenylmethane, 3,3xe2x80x2-diaminodiphenylmethane, 4,4xe2x80x2-diaminodiphenyl sulfide, 3,3xe2x80x2-diaminodiphenyl sulfide, 4,4xe2x80x2-diaminodiphenyl sulfone, 3,3xe2x80x2-diaminodiphenyl sulfone, 4,4xe2x80x2-diaminodiphenyl ether, 3,3xe2x80x2-diaminodiphenyl ether, benzidine, 3,3xe2x80x2-diaminobiphenyl, 3,3xe2x80x2-dimethyl-4,4xe2x80x2-diaminobiphenyl, 3,3xe2x80x2-dimethoxybenzidine, bis(p-aminocyclohexyl)methane, bis(p-xcex2-amino-t-butylphenyl) ether, bis(p-xcex2-methyl-xcex4-aminopentyl)benzene, p-bis(2-methyl-4-aminopentyl)benzene, 1,5-diaminonaphthalene, 2,6-diaminonaphthalene, 2,4-bis(xcex2-amino-t-butyl)toluene, 2,4-diaminotoluene, m-xylene-2,5-diamine, p-xylene-2,5-diamine, m-xylylenediamine, p-xylylenediamine, 2,6-diaminopyridine, 2,5-diaminopyridine, 2,5-diamino-1,3,4-oxadiazole, 1,4-diaminocyclohexane, piperazine, methylenediamine, ethylenediamine, tetramethylenediamine, pentamethylenediamine, hexamethylenediamine, 2,5-dimethylhexamethylenediamine, 3-methoxyhexamethylenediamine, heptamethylenediamine, 2,5-dimethylheptamethylenediamine, 3-methylheptamethylenediamine, 4,4-dimethylheptamethylenediamine, octamethylenediamine, nonamethylenediamine, 5-methylnonamethylenediamine, decamethylenediamine, 1,3-bis(3-aminophenoxy)benzene, 2,2-bis[4-(4-aminophenoxy)phenyl]propane, 1,3-bis(4-aminophenoxy)benzene, bis-4-(4-aminophenoxy)phenyl sulfone, bis-4-(3-aminophenoxy)phenyl sulfone, 9,9xe2x80x2-bis(4-aminophenyl)fluorene, and 2,2-bis(4-aminophenyl)hexafluoropropane, or siloxanediamines such as xcex1,xcfx89-bis(3-aminopropyl)polydimethylsiloxane. These diamines can be used each alone or in combination of two or more.
In addition to the diamine and at least one tetracarboxylic acid component selected from the group consisting of a tetracarboxylic acid, a tetracarboxylic acid monoanhydride and a tetracarboxylic acid dianhydride, capable of forming two imide rings upon cyclization, there may be added a dicarboxylic acid and a dicarboxylic acid anhydride such as phthalic acid and phthalic anhydride or a monoamine such as aniline in a small amount for the purpose of controlling the molecular weight and maintaining processability. Furthermore, as far as performances are not damaged, various additives such as a filler can be simultaneously added.
The synthesis of polyimide resins according to the present invention is carried out by mixing the above tetracarboxylic acid component and the above diamine at a molar ratio of 0.8-1.2:1, and heating and reacting the components at 80-450xc2x0 C. without adding a solvent to the mixture, namely, in the solventless state, to form an imide compound. The tetracarboxylic acid component and the diamine used in this invention are generally solid, and a solid mixture is obtained by mixing them as they are. There are liquid monomers such as a siliconediamine, but when these are mixed in liquid state with the solid tetracarboxylic acid component without adding solvents, solid mixtures are also obtained.
The method for mixing the tetracarboxylic acid component with the diamine in the present invention is not limited as far as the solids can be uniformly mixed, and it is preferred to mix them as uniformly as possible using a mortar or a high speed stirring mixer provided with a heating means. The heat-treating temperature is generally 80-450xc2x0 C. during a heating time of 1 minute to 30 hours, but it is necessary to determine an optimum temperature in accordance with proceeding of the reaction depending on a combination of the monomers. For example, the temperature can be gradually raised from a low temperature, or the temperature can be raised stepwise in such a manner as 80xc2x0 C./30 minutes+130xc2x0 C./1 hour+200xc2x0 C./1 hour. If the heating temperature is too low (generally, lower than 80xc2x0 C.), the reaction rate abruptly decreases to leave unreacted substances, and resins of high molecular weight cannot be obtained. If the heat treatment is carried out at a temperature higher than the heat decomposition temperature of the polymer, undesirable side reactions such as heat deterioration and crosslinking take place to often damage the subsequent processability or properties of the resin.
The polyimide resin thus obtained can be ground and used for molding as it is, and when the polyimide resin is soluble in a solvent, the resin is dissolved in the solvent to prepare a resin varnish and this can be used in the same manner as in the conventional polyimide resin varnishes.
The present invention will be explained in more detail by the following examples. These examples should not be construed as limiting the invention in any manner.
Methods and conditions for measurement of various properties of the polyimide resins obtained by the present invention are as follows.
(1) Molecular weight distribution:
Using a gel permeation chromatography (GPC) device (a high performance liquid chromatogram manufactured by Waters Co., Ltd. to which is connected a polystyrene column GL-S300MDT-5 manufactured by Hitachi Chemical Co., Ltd.) with tetrahydrofuran/N,N-dimethylformamide/phosphoric acid(100/100/1 in weight ratio) as a mobile phase, an absorbance for 270 nm was measured by Model 484 absorbance meter manufactured by Waters Co., Ltd., from which molecular weight and molecular weight distribution (in terms of polystyrene) were calculated.
(2) Glass transition temperature and melting point:
These were measured using a differential scanning calorimeter (DSC220C manufactured by Seiko Denshi Kogyo Co., Ltd.) under the conditions of a heating rate of 10xc2x0 C./min and a temperature range of 30-500xc2x0 C.